Mobile radio communications presently mainly rely on conventional whip-type antennas mounted to the roof, hood, or trunk of a motor vehicle. Although whip antennas generally provide acceptable mobile communications performance, they have a number of disadvantages. For example, a whip antenna must be mounted on an exterior surface of the vehicle, so that the antenna is unprotected from the weather, and may for example, be damaged by vehicle washes, unless temporarily removed.
The user of mobile radio equipment is often plagued today by the problem of vandalism of car radio antennas and burglary of the equipment. Indeed, the presence of a whip antenna on the exterior of a car is a good clue to thieves that a radio, telephone transceiver or other equipment is installed within the vehicle.
Varieties of covert antennas are known in the art. Such antennas are usually substantially flush-mounted to a vehicle, covered with fiberglass and refinished to blend with the rest of the car body. In particular, annular slot-type stripline antennas can be useful, as where such an antenna is to be substantially flush-mounted to a vehicle. One such annular slot-type stripline antenna element is described in U.S. Pat. No. 3,665,480. As discussed therein, the antenna element includes a pair of parallel conductive plates formed on opposite faces of a dielectric support structure, one of which has formed therein a generally annular radiating slot of substantially uniform width, and a feed element disposed between the parallel plates and extending radially into the central region of the annular slot for feeding electromagnetic energy into such a slot.
U.S. Pat. No. 4,821,040 describes a compact quarter-wavelength microstrip element especially suited for use as a mobile radio antenna. The antenna is not visible to a passerby observer when installed, since it is literally part of the vehicle. The microstrip radiating element is conformal to a passenger vehicle, and may, for example, be mounted under a plastic roof between the roof and the headliner.
U.S. Pat. No. 4,821,042 describes a vehicle antenna system including high frequency pickup type antennas concealed within the vehicle body for receiving broadcast waves. The high frequency pickups are arranged on the vehicle body at locations spaced apart from one another, that is, at least one adjacent to the vehicle roof and the other on a trunk hinge.
U.S. Pat. No. 5,402,134 describes a flat plate antenna module for use in a non-conductive cab of a motor vehicle and includes a dielectric substrate and one or more antenna loops arranged on the substrate. The substrate is adapted to be installed between the headliner of a cab and the dielectric roof. The module may include a CB antenna loop, an AM/FM antenna loop, a cellular mobile telephone antenna loop, and a global positioning system antenna, without the need for any antenna structure external to the cab. The antennae are arranged on the module in a nested configuration.
U.S. Pat. No. 6,023,243 describes a flat panel antenna for microwave transmission. The antenna comprises at least one printed circuit board, and has active elements including radiating elements and transmission lines. There is at least one ground plane for the radiating elements and at least one surface serving as a ground plane for the transmission lines. The panel is arranged such that the spacing between the radiating elements and their respective groundplane is independent of the spacing between the transmission lines and their respective groundplane. A radome may additionally be provided which comprises laminations of polyolefin and an outer skin of polypropylene.